The present invention relates to controllers employing rate feedback loops, and more particularly to digital servo controllers employing a means for substantially reducing velocity quantization errors under low rate or velocity conditions, thereby reducing noise, allowing a more robust operation, and eliminating the need for a dynamically reconfigured servo controller.
It is well known in the art to employ rate feedback loops or compensation in controllers as a means for improved controller performance. An exemplary description of rate feedback compensation appears in "Linear Control System Analysis and Design," John J. D'Azzo and Constantine H. Houpis, 1975, McGraw-Hill, Inc., at 10-14 et seq. FIG. 1 shows an exemplary block diagram of a conventional controller for the controlled plant 25, which may typically comprise a servo motor. Here, a reference signal is provided to summing element 20 which combines the reference signal with the inverted contributions from the position feedback loop 30 and the rate feedback loop 35. The position feedback loop 30 provides a contribution dependent on the position of the controlled plant 25, as amplified by the loop position constant K.sub.1. The rate feedback loop 35, analyzed in terms of the Laplace transform S plane, converts the position data from the plant 25 into rate or velocity data by multiplication by S, and then amplifies the rate data by the loop rate constant K.sub.2. The resultant combined signal is then provided to the controlled plant 25 as the drive signal. The use of rate compensation offers significantly improved performance over controllers employing only position feedback compensation.
It is also known to digitize the conventional rate compensation controller of FIG. 1. Typically, in the application of a servo motor controller, the position and rate data are provided by a motor shaft encoder. The encoder provides a pulse each time the shaft rotates through a predetermined angular excursion. For example, optical encoders use a disc having a plurality of radially aligned slots disposed around the periphery of the disc. A light source is trained on the periphery of the disc on one side thereof with a light detector disposed on the other side thereof. As the disc rotates with the shaft, a series of pulses will be generated by the detector as the encoder disc interrupts the light beam. Thus, the encoder has a predetermined angular resolution dependent on the spacing of the slots on the disc periphery. This angular resolution translates into some translational resolution, in dependence on the particular mechanism driven by the motor. For a digitized controller, each loop is conventionally operated at a particular sample rate, i.e., the contribution from each loop is updated based on fresh estimates of the position and rate once each sample period.
One typical application of a servo controller with rate compensation is for X-Y plotters, using an X-Y positioning apparatus to position a plotter pen and the plotter paper. These devices employ two servo motors, one for each axis, and the controller for each motor may employ rate compensation. Under high velocity, i.e., high rate, conditions for a particular motor or axis, the conventional controller performs satisfactorily. The encoder resolution is not a severe problem, since the angular excursion of the motor shaft between samples will be many times the minimum resolution of the encoder, so that the possible measurement error is a relatively small percentage of the total excursion. Under low velocity conditions, however, the quantization error resulting from the encoder operation is a serious problem, since the same sample rate is conventionally employed for the low velocity as well as the high velocity conditions. Thus, the motor shaft may have moved only very slightly between samples, so that the position and rate estimates or calculations are inaccurate as a result of the quantization error when the encoder provides a new pulse or incremented output. Such errors result in audible noise or jitter of the plotter apparatus under stationary conditions, reducing the robustness of operation, and may require a dynamically reconfigured servo or a higher resolution encoder to provide satisfactory performance, at substantially increased expense.
Others are believed to have sought to improve the performance of rate compensation controllers by varying equally the sample rates of both the position and rate feedback loop at low velocities, or by operating the two loops at fixed, but different sample rates, e.g., operating the position loop at a 1 Khz sample rate and the velocity loop at 5 Khz. These approaches are not believed to provide a satisfactory solution to the problem, however.
It is therefore an object of the present invention to provide a servo controller employing rate compensation but which substantially eliminates rate quantization error under low velocity conditions while not requiring expensive high resolution encoders or complex dynamically reconfigurable controllers.